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Wi-Fi
This page covers the practical aspects of wireless access to the Internet via Wi-Fi. Introduction to Wi-Fi Wi-Fi Wireless Networks provide all the functionality of wired networks, without the need for physical connections. Data is sent over various radio frequencies depending on the service. Typical Wi-Fi connection speeds are 11Mbps and 54Mbps. In practice, throughput is about half the connection speed. Wireless Networks basically consists of computers equipped with wireless NICs, which may be USB, PCI, MiniPCI, CF card, PCMCIA, CardBus, or other interface. Connectivity to a wired network requires an access point (AP) used as a bridging device. Connectivity to the internet usually requires a wireless router, which includes an access point. AP's are typically located near the centre of the wireless client concentration. Wi-Fi Network Types Infrastructure Ad Hoc Wi-Fi Hardware Wireless Router : A wireless router consists of three sections. A wireless access point (radio), an ethernet router, and an ethernet switch. Some boxes include either an ADSL modem or a cable modem. Wireless Access Point : A wireless access point is the central wireless bridge device use in an infrastructure network. Traffic from the wireless side of the bridge is sent to the ethernet side of the bridge. The wireless access point also controls all traffic between wireless client radios. Wireless Bridge All 802.11 wireless is bridging, where everything is done on Layer 2. IP addresses are used only for configuration of the access points. Wi-Fi encapsulates 802.3 ethernet packets inside 802.11 wireless packets. * Single MAC address client adapter : Common client radio used in workstations, laptops, and PDA's. Limited to connecting one computer to an infrastructure access point. * Multiple MAC address client adapter : Client wireless adapter that can bridge more than one MAC address. Used for connecting more than one computer via a single wireless link to an infrastructure access point. * Point to point wireless bridge : Two identical radios used to connect two networks. Same as transparent bridge. Will bridge more than one MAC address. * Transparent bridges. : Bridges more than one MAC address. Same as point to point bridge. * Point to multipoint wireless bridge : Three or more identical radios used to connect multiple networks. Bridge more than one MAC address * Workgroup bridge : Bridges a limited number of MAC addresses (usually 4, 8, or 16) to an infrastructure access point. 3Com 3CRWE675075, 3CRWE83096A, or Cisco AIR-WGB352. * Game adapter : Muddled marketing term for either a workgroup bridge or single MAC address client adapter. Bridges an indeterminant number of MAC addresses. Some do one MAC, some do more. Good luck. * Wireless repeaters : Store and forward repeater. Repeats all packets it hears for a configured SSID. Also repeats all broadcasts. Used to extend the range or coverage of a wireless network. * WDS bridge : Simultaneously act as a bridge and as an access point. Used to extend the range or coverage of a network. Wireless users can connect to any WDS bridge as if were an access point. WDS bridge radios talk to each other as if they were transparent bridge radios. * Wireless ethernet switch : Multiple simple access point radios connected to a central intelligent switch. Offers centralized management and monitoring. Very easy to expand. Note that a switch is a bridge with more than two ports. Adapters * PCI * USB * PC Cards Link Calculations The easiest way to do this is by example. Your setup and numbers will vary. This is the best case solution, with no consideration for atmospherics, Fesnel Zone edge diffraction, folliage attenuation, and non-line of sight effect. Fade margin only gets worse, never better. We'll use a pair of Linksys WRT54G v4 802.11g with DD-WRT alternative firmware setup as a transparent bridge. This makes it easy because both sides are the same. The transmit output is about +17dBm. The WRT54G will be mounted in a weatherproof box somewhere near the antenna with a single 3ft cable (pigtail) using LMR-240 coax with a loss of about 4dB including connectors. Next, we gotta do some guesswork. I'll assume that we can live with about 6Mbits/sec thruput. That means the connection speed will be about twice that or 12Mbits/sec. The receiver sensitivity varies with the speed and modulation type. The following was lifted from the Dlink DI-624 datasheet but is close enough for most similar 802.11g radios: | | |} :12Mbps connect speed thus sets the receiver sensitivity at -84dBm. :PER (Packet Error Rate) is similar to BER (Bit Error Rate) but is easier to measure. You would not want to operate at a 10% PER error rate. 1 in 10 packets trashed is useable, but no fun. This is approximately the same as a BER of 1E10^5 which is one of the common reference levels for measuring receiver sensitivity. Again, these are measurement reference levels, not operating points. Next, we must guess the fade margin. Fade margin or system operating margin is how much stronger the operating level is above the receiver reference level. 20dB is considered to be a good minimum. In other words, your operating signal level must be 100 times stronger than the receiver sensitivity or the link is going to be flaky and unreliable. The relationship between fade margin and reliability is: Reliability Fade Margin 90% 8 dB 99% 18 dB 99.9% 28 dB 99.99% 38 dB 99.999% 48 dB 99% reliability might sound great, but that means your link will be useless for 1% of the year, or 3.6 days per year. Don't go below 20dB fade margin. Plugging the above guesswork into a suitable Link Calculator and trying various antenna gains until we get a 20dB fade margin: TX power = +15dBm TX coax loss = 4dB (3ft LMR-240 plus a mess of connectors) TX ant gain = unknown Distance = 2 miles RX ant gain = unknown RX coax loss = 4dB (same at other end) RX sens = -84dBm (at 12Mbits/sec) Fade margin = 20dB yields a minimum antenna gain of +21dBi. Any less than +21dBi antenna gain will result in insufficient fade margin and a corresponding loss in link reliability. That makes the required antenna to be a dish. We can use a lower gain antenna by running at a slower speed, shorter coax, or more transmit power at both ends. Online link calculators: http://www.terabeam.com/support/calculations/som.php http://my.athenet.net/~multiplx/cgi-bin/wireless.main.cgi http://www.connect802.com/antenna_c_main.php Performance and Speed Wireless connecton speed and thruput are quite different. The wireless may offer a connection speed of 54Mbits/sec, but the actual thruput is considerably less, typically less than half. That's because of management packet overhead, 802.11b compatibility, inter-symbol gaps, and necessary timing delays. Also, note that wireless is half-duplex, where only one radio in a given airspace, can transmit at a time. This is stolen from an Atheros paper with some additions and corrections. Non-overlapping Modulation Max Max Max Channels Link TCP UDP 802.11b 3 CCK 11 5.9 7.1 802.11g (with 802.11b) 3 OFDM/CCK 54 14.4 19.5 802.11g only 3 OFDM 54 24.4 30.5 802.11g turbo 1 OFDM 108 42.9 54.8 802.11a 13 OFDM 54 24.4 30.5 802.11a turbo 6 OFDM 108 42.9 54.8 The paper claims that encryption is enabled for these calculations, but the numbers seem to indicate that these number are for encryption disabled. I don't know for sure. The maximum TCP and maximum UDP are the theoretical maximum thruput rates. No calculations Turbo and SuperG modes yet. There is also a relationship between speed an range. The following table is from a paper by Intel on wireless hotspot deployment for 802.11b/g. Rate Range Rate Range Mbps ft Mbps ft 1 350 12 200 2 250 18 170 5.5 180 24 140 6 300 36 100 9 250 48 95 11 150 54 90 The rather optimistic ranges listed are probably with a better than standard omnidirectional antenna and with unobstructed line of sight. There are similar charts in the Intel hot spot paper for other frequencies and protocols. Repeaters Antennas : See Wifi:Building an antenna Manufacturers * Belkin * D-link * Linksys (part of Cisco Systems) ** Linksys.com Home Page ** Linksysinfo Good source of information on Linksys products * NETGEAR Software Wi-Fi Security : See: :* WiFi:Security :* The Unofficial 802.11 Security Web Page :* Simple advice for securing your home wireless LAN Wi-Fi Security Myths :* The six dumbest ways to secure a wireless LAN (Wireless LAN security hall of shame) :** MAC filtering (absolutely worthless) :** SSID hiding (doesn't really work, causes needless grief) :*** Debunking the Myth of SSID Hiding (registration required) :** LEAP authentication (can be rapidly cracked) :** Disable DHCP (waste of time) :** Antenna placement (hackers have better antennas than you do) :** Just use 802.11a or Bluetooth (have nothing to do with security) :* Dispelling the Myth of Wireless Security Intruder Detection :* AirSnare WEP WPA SSL VPN VPN Service Providers VPN which stands for Virtual Private Network,that is used to construct and connect private networks using the public resource called Internet.the communications between these networks occurs in a more secret way using diffeerent security methods like WEP,WPA,WPA2,L2TP,PPTP based on the type of VPN.In general there are two types of VPNS are available:first,Remote -access VPN in which both payload and data are encrypted and send while in Site-to-Site VPN ,in which only data are encrypted and send as a private data through the Internet. :* HotSpotVPN (commercial) :* iPIG (free) :* JiWire SpotLock (commercial) :* PublicVPN.com (commercial) :* WiTopia (commercial) What Makes for a Good Password or Passphrase? : See Password strength : See also: :* Password :* Passphrase :* Diceware Passphrase Home Page :* Password Safe (generates and stores passwords securely) What is Wardriving? : See Wifi:Wardriving Wi-Fi Hotspots See Wifi:Setting up a hotspot Free Public HotSpot Directories * JiWire * MetroFreeFi * Wi-Fi-FreeSpot Directory * WiFinder * Australian HotSpots Commercial Hotspots * Directories ** JiWire ** WiFinder * Networks ** Airpath ** at&t (SBC) FreedomLink ** Boingo ** iBAHN ** ICOA ** iPass ** NetNearU ** Sprint ** T-Mobile ** Wayport ** WeRoam Special Wi-Fi Applications Marine :* Wi-Fi Internet solutions for boaters and marinas :* MarineNet Wireless :** WiFi Antenna Installations Streaming Audio to Home Stereo :* Media Adapter with computer as server: :** Wired Media Adapter with a Wi-Fi-Ethernet bridge/gaming adapter (e.g., Hauppauge MediaMVP with Linksys WRT54G as client bridge) :** Wireless Media Adapter (e.g., Apple Airport Express driven by Apple iTunes software) :* Standalone Internet radio connecting to Wi-Fi network: :** Wired Internet radio with a Wi-Fi-Ethernet bridge/gaming adapter (e.g., Slim Devices Squeezebox with Linksys WRT54G as client bridge) :** Wireless Internet radio Troubleshooting Wi-Fi Interference :* Checklist of possible interference sources: :** Microwave oven :** 2.4Ghz video or security camera link (X10). :** Municipal wireless network. :** Bluetooth devices (mouse, phone, digital assistant PDA, headset, cell phone, etc.) :** Portable wireless TV camera used at sports events. :** spread spectrum Frequency hopping cordless phones (e.g., Panasonic Gigarange) :** Wi-Fi (IEEE 802.11b/g) wireless keyboard, digital assistant PDA, and/or cell phone. :** 2.4GHz wireless gamepad and/or controller. :** RF Excited Lighting (e.g., Fusion Lighting). See Part 18 RF Lighting, A Potential "Extinction Level Event" For Communications Users Of The 2.4 GHz Band :** 2.4GHz wireless baby monitor. :** 2.4GHz ham radio operation. :** WISP (Wireless Internet Service Providers) which may be using non-Wi-Fi type of modulation (e.g., WiMAX). :** Breezecom/Alvarion/Symbol/Raylink spread spectrum frequency hopping networks. :** Western Multiplex or Proxim non-Wi-Fi wireless links (e.g., Lynx). :** HomeRF spread spectrum frequency hopping network. :** ZigBee IEEE 802.15.4 sensor wireless network. :** Microwave fruit drying oven, plastic mold preheater. :** Unstable high power Wi-Fi power amplifier spewing RF "garbage". :* Resources ::* Minimizing 802.11 Interference Issues ::* Stop the interference! :: Note:- 2.4GHz phones are not an issue in the UK(Eur) as DECT phones operate from 1.88GHz to 1.9GHz. :* Checking for Wi-Fi Networks ::* Network Stumbler ::* Kismet ::* Hitchhiker :* Non-interfering Wi-Fi Channels :: Nearby Wi-Fi channels overlap, and signals on two different channels can interfere with each other when they are less than 5 channels apart, with the amount of interference decreasing with channel separation. (Thus channels 1 and 3 interfere less with each other than channels 1 and 2.) For minimum interference between different Wi-Fi networks, use Channels 1, 6, and 11, which don't overlap. Poor Signal : See Antennas. SSID Conflict : When two networks have the same SSID, wireless devices on one network may try to connect to the other network (because they look like the same network), which can cause problems. This can easily happen when two differnet networks use hardware from the same vendor with the same default SSID. To avoid such problems, configure a unique SSID for your network that won't conflict with other networks. External Links * WiFi-Forum * WiFi Resources